We are studying the factors affecting peripheral venous capacity. We measure esophageal temperature (Tes), skin temperature (Tsk) and forearm venous volume (FVV). Forearm volume is measured with a Witney strain gauge, and FVV is taken as the difference between volume of the arm filled to a standard congesting pressure and volume measured with the veins emptied by gravity. Experiments are conducted under a variety of conditions of thermal and circulatory stress. We have developed a basic model, describing control of FVV as a function of Tes and skin temperature at rest and at one moderate level of exercise. We will extend this model to include effects of non-thermal stresses on the circulation, such as factors which impair venous return. Such a model will contribute to a more complete understanding of the mechanisms involved in heat syncope and other heat disorders, in which venous pooling plays a prominent etiological role. Studies of FVV will be supplemented with similar studies of calf venous volume, to test how well FVV reflects cutaneous venomotor activity elsewhere on the body. We will study the effect of local temperature by comparing bilateral measurements of FVV on forearm maintained at a local temperature different from each other. At several levels of venomotor activity, we will study venous compliance by varying the congesting pressure and measuring venous volume, and pressure within the forearm veins. Having a model describing the relation of peripheral venous volume to Tes, mean skin temperature, local temperature, and exercise intensity, we will investigate how parameters of the model are affected by a program of exercise training and heat acclimation. We will also investigate the role of the veins in core to skin heat transfer by determining the relationship between thermal conductance and skin blood flow at several different levels of venomotor tone. In addition, we will determine the effect of increased peripheral vascular pooling on stroke volume, which affects limit cardiac output under conditions of heat strain. Additional work in this project includes investigating the effects of such factors as age, sex, and season on the control parameters, which will aid prediction of individual tolerance to heat stress.